Many species of bacteria swim using flagella, which consist of thin helical filaments turned by rotary motors in the cell membrane. Assembly of the bacterial flagellum depends on a specialized secretion apparatus at the base that exports the protein subunits needed to form exterior structures. The flagellar secretion apparatus obtains energy from the membrane ion gradient, and transports protein subunits more rapidly than other known export systems. At the center of the apparatus is a complex formed by the membrane proteins FliP, FIiQ, FIiR, FlhA, and FlhB. The mechanism of flagellar secretion is poorly understood, mainly due to the lack of information on this membrane-protein complex. The proposed work will use a variety of genetic and biochemical approaches to determine which proteins are essential for export, to elucidate their membrane topologies, and to identify functionally important domains and residues.